The present invention relates in general to communication systems, and is particularly directed to a new and improved embedded operations channel (EOC)-based signalling mechanism, that is incorporatable within transceiver interface (U-BR1TE) components of an integrated services digital network (ISDN) telecommunication system, for effecting a relatively fast, warm start reactivation of a deactivated extended ISDN link between an ISDN switch and customer premises equipment.
Integrated services digital network (ISDN) communication systems enable telecommunication service providers to supply multiple types of signalling channels from a central office over a signal local loop twisted pair to a network termination interface at a customer premises site. ISDN signalling channels, for example, typically include digital data and/or digitized voice (bearer B1/B2) channels, as well as a separate administrative (D) channel that conveys call control information. FIG. 1 shows a reduced complexity example of a non-extended distance ISDN communication system, in which ISDN communications are provided over a local (two-wire metallic) loop 13 directly connecting a central office site (CO) 10 to a customer premises site 20. FIG. 2 diagrammatically illustrates a typical extended distance ISDN communication system, wherein ISDN connectivity between the CO site 10 and a remotely located customer premises site 20 is provided over an extended (PCM) communication link (such as a fiber optic link) 30.
In each of these two ISDN networks, the central office site 10 includes a central office switch 11 (such as a 5ESS switch manufactured by ATandT), which contains a plurality of ISDN transceiver circuits (ISDN U-interface circuits), one of which is shown at 12. For the non-extended distance ISDN system of FIG. 1, the ISDN U-interface circuit 12 terminates one end of a local loop (twisted tip/ring pair) 13, a second end of which ported to a network termination interface (NT-1) circuit 21 serving customer premises equipment, such as ISDN data terminal equipment (DTE) 22.
In the extended distance system of FIG. 2, a U-interface circuit 12 of the central office site 10, located at a xe2x80x98westxe2x80x99 end of extended (PCM) communication link 30, is coupled over the local loop 13 to what is termed in the telephone industry as a U Basic Rate-One Transmission Extension (U-BR1TE) transceiver card 14 (also referred to as a Basic Rate Interface Transmission Extension (U-BRITE) card).
A respective U-BR1TE (or U-BRITE) card contains an ISDN U-interface circuit and an associated PCM bus interface which are operative to transmit and receive standard 2B+D ISDN data traffic over a PCM digital data link, such as a T1=1.544 MB/s link, and to interface the ISDN signals via the local loop to and from loop termination equipment. Namely, the U-BR1TE card 14 at the xe2x80x98westxe2x80x99 end of the link 30 interfaces the local loop 13 with a digital data link 30 for PCM signalling transport to another U-BR1TE transceiver card 24 at an xe2x80x98eastxe2x80x99 end of the link 30, serving the customer premises equipment (CPE) site 20. The xe2x80x98eastxe2x80x99 U-BR1TE transceiver card 24 is coupled, in turn, over a local loop (twisted two-wire tip/ring pair) 23 to the network termination interface (NT-1) circuit 21, to which customer premises ISDN data terminal equipment 22 is coupled.
A typical ISDN U-interface circuit is capable of operating in what is commonly referred to as a xe2x80x98warmxe2x80x99 start mode, which allows the loop to be deactivated and then, relatively quickly reactivated at a later time, by using an abbreviated training sequence that customarily requires up to 300 msec, rather than up to as many as fifteen seconds in what is commonly referred to as xe2x80x98cold startxe2x80x99 mode. Only the ISDN switch can command deactivation of the link, while either the ISDN switch or the customer premises NT-1 can initiate reactivation of a previously deactivated loop (by the transmission of a 10 kHz wake-up tone).
In order to initiate a warm start deactivation sequence, the ISDN switch 11 sets a specific xe2x80x98downstream onlyxe2x80x99 maintenance bit in an overhead channel to the NT-1 circuit 21. While this procedure is straightforward forward in the non-extended network of FIG. 1, it becomes a problem in the extended network of FIG. 2, as there is no defined scheme to signal reactivation across an extended interface between the ISDN switch and the NT-1. To address this problem, Bellcore publication TR-NWT-000397 proposes waking up, or reactivating, an extended link by toggling the state of the ACT bit within the M4 out-of-band maintenance channel alternately between opposite logic levels.
In particular, the ACT bit is transmitted low for three sequential superframes, high for the next three superframes, and then low for the next three superframes. Upon detecting this ACT bit toggling sequence, the U-BR1TE card 24 at the receiving end of the extended link supplies the 10 Khz wake-up tone over the local loop 23 to the NT-1. A problem with this approach is the significant delay involved (nine superframesxc3x97twelve msec per superframe=108 msec of additional delay in bringing up both loops), which may cause layer two and layer three signaling errors.
In accordance with the present invention, this multiple superframe-based delay problem in reactivating an extended ISDN link is successfully addressed by means of a communication control software mechanism, which utilizes the embedded operations channel (EOC) to convey a relatively fast xe2x80x98wake-up the loopxe2x80x99 message (that consumes only six msec of delay) between U-BR1TE circuits terminating the extended distance loop. In response to such a xe2x80x98wake-up the loopxe2x80x99 message, the receiving U-BR1TE card proceeds to provide a 10 Khz wake-up tone to its associated termination equipment (either to the NT-1 in the case of a switch-sourced reactivation, or to the ISDN switch in the case of an NT-1 sourced wake-up request), thereby waking up the loop.